In recent years, with the advance of digital technologies, electronic devices, such as mobile information devices and information home appliances, have been provided with higher functions. There have therefore been increased demands for an increased capacity of a memory device, reduction of writing power, a higher speed of writing and reading, and a longer lifetime.
It is said that miniaturization of a flash memory including existing floating gates has a limit and fails to meet these demands. On the other hand, a variable resistance nonvolatile memory element having a variable resistance layer serving as a memory unit includes a variable resistance element having a simple structure sandwiched by a pair of electrodes. Therefore, it is expected that such a nonvolatile memory element has a possibility of further miniaturization, a higher speed, and further consumption power saving.
If a variable resistance layer serves as a memory unit, for example, application of an electric pulse or the like changes a resistance value of the variable resistance layer from high to low or from low to high. In this case, it is necessary to clearly distinguish between the two resistance values in low resistance and in high resistance, at the same time, stably change between low resistance and high resistance at a high speed, and then hold the two resistance values in the nonvolatile manner. In order to stabilize the memory characteristics and miniaturize the memory element, there have conventionally been various propositions.
For example, Patent Literature 1 discloses a nonvolatile memory element in which a variable resistance layer is provided between a pair of electrodes. The variable resistance layer has a multi-layer structure including a first variable resistance layer and a second variable resistance layer which comprise the same kind of transition metal oxide having different oxygen content atomic percentages. According to Patent Literature 1, the use of the nonvolatile memory element makes it possible to selectively cause oxidation-reduction reaction near an interface between an electrode and a variable resistance layer having a high oxygen content atomic percentage and, thereby providing stable resistance change.